Entangled Quanta Beat Decoherence

LOS ALAMOS, N.M. -- As the feasibility of creating computers and communications systems based on quantum entanglement becomes clear, a problem remains: Environmental forces can disrupt their spooky nonlocal correlations. A team of researchers at Los Alamos National Laboratory has demonstrated experimentally, however, that some entangled states are immune to this interference, pushing the realization of these technologies even nearer.

"One of the great things about entangled systems is that they can have built-in protection from environmental noise," said Paul G. Kwiat, formerly a research physicist at the laboratory and a member of the team, which presented its findings in the Oct. 20, 2000, issue of Science.

Interactions with the environment through which entangled quanta are traveling can induce unwanted couplings that destroy the correlation and so compromise their practical use in quantum computing or communications. Luckily, quantum mechanics predicts that there should be particular entangled states that are largely resistant to these environmental forces, residing in so-called decoherence-free subspaces.

"For a decoherence-free subspace, each qubit [quantum bit] is subject to the same disturbances, and there is a special combination that does not display decoherence," Kwiat explained. "The classical analogy is subtracting out noise from a common-mode-rejection amplifier. If the noise is the same in both inputs, it cancels out."

To verify the existence of these subspaces, the researchers monitored the polarizations of entangled photons that had traveled through 10-mm-thick pieces of quartz.

They created the pairs using the spontaneous parametric down-conversion of 351-nm photons from an 80-mW argon-ion laser in adjacent nonlinear optical crystals, and half-wave plates prepared the photons in four entangled states. Adjustable quarter- and half-wave plates and polarizing beamsplitters placed in front of silicon avalanche photodiodes enabled the team to tomographically reconstruct the states. In accordance with theory, the correlations in one state were largely unaffected by the decohering pieces of quartz.

Subspace applications

Kwiat, now a professor at the University of Illinois in Urbana-Champaign, noted that decoherence is generally not a problem for entangled photons but that the researchers chose to work with them in this experiment because they yield cleaner results. Applications involving other entangled systems, such as in linear ion trap quantum computation, also may benefit.

There are systems "in which deleterious environmental conditions act pretty much the same on all [qubits] with a few perturbations," he said. "In this case, you can use decoherence-free subspaces to combat most of the decoherence and active error correction for the rest."